Field of the Invention
This invention pertains to the general field of optical metrology. In particular, the invention relates to a method for profiling the bottom surface of vias in printed circuit boards when the view is partially obscured by preferentially oriented reinforcement fibers added to the structure of the boards.
Description of the Prior Art
Printed circuit boards (PCBs) are well known structures used in the electronic industry to mount electronic components and connect them to external devices. Printed circuit boards have traditionally been manufactured from fiber layers surrounded by a plastic matrix material. The boards have one or more layers of metalized patterns that, when assembled with the electronic components, form electrical interconnections among them.
In use, assembled PCBs are normally attached to a chassis, such as the frame of a computer, and are therefore subjected to stresses due to vibrations and to forces exerted by the weight of the components attached to them. These forces tend to produce undesirable flexing of the circuit boards with attendant potential loosening of the electrical connections and separation of the electronic components. Therefore, in order to minimize flexing, it has become common practice to reinforce PCBs by means of support structures such as reinforcing bars, beams and rib stiffeners. However, such support structures are often undesirable because they occupy valuable circuit-board surface area, which is contrary to the trend of increasing the density of electrical components on PCBs. Moreover, electrical components are becoming increasingly heavy, thus placing an increased burden on the structure of the PCB.
In addition, ever increasing miniaturization requirements tend to lead to thinner and thinner PCBs, which therefore are also more flexible and more subject to potential damage. U.S. Publication No. 2004/0131824 provides a solution to this problem by reinforcing and stiffening the structure of printed circuit boards in selected locations using preferentially oriented fibers. Selected fibers are removed from the polymeric material matrix of the PCB and replaced with a similar quantity of different-material fibers placed in a predetermined orientation as required to achieve the desired PCB stiffening. Because printed circuit boards tend to flex along a particular axis, the reinforcing fibers are oriented transversely to resist flexure, thereby reducing material fatigue, fracture and failure. FIG. 1 illustrates a section 10 of PCB polymeric matrix where such reinforcing fibers 12 are shown laid cross the structure of the PCB. This reinforcement approach has become common practice in the industry.
During the process of assembly of electronic components to the PCB, holes 14 (referred to as “vias” in the industry) are typically drilled with lasers into the PCB matrix, as shown in FIG. 2, for receiving and soldering the leads or pins of chips and other components. The vias are metalized to form an electrical connection between the electrical component pins inserted into them and the circuit board. Therefore, the vias have to be large and uniform enough to make a good and strong connection between the printed circuit and the electronic components when the vias are filled with soldering material. To that end, knowledge of the exact dimensions of the vias is a critical part of the packaging process and the top and bottom diameters of the vias are measured routinely for quality control purposes.
Various optical systems and techniques are known that could be used to profile vias. These include, without limitation, low-coherence interferometry, confocal microscopy, bright-field and dark-field microscopy (image sharpness techniques), and structured light techniques. These methods are all encompassed by what is generally referred to in the art, interchangeably, as optical metrology, optical profilometry, or 3-D microscopy. The signal captured in low-coherence interferometry (including structured light metrology) is fringes, while in confocal microscopy, bright-field microscopy and dark-field microscopy the optical signal is irradiance.
When the dimensions of the via are measured with low-coherence interferometric (WLI) techniques, the via is scanned with an interferometric objective with a field of view exceeding the top aperture of the via (that is, an objective overlying the entire via opening or a system adapted to cover that area through stitching of data acquired with high numerical-aperture objective and a smaller field of view) and the top and bottom surfaces are profiled by identifying in conventional manner the scanning heights where local fringe modulation maxima are produced during the scan through focus. However, it has been found that drilling vias in PCBs reinforced with oriented fibers does not produce uniform tubular structures because the reinforcing fiber material tends to melt away at a different rate than the PCB matrix when drilled with a laser and leave behind loose fibers that form a substantially annular shelf 16 that protrudes into the vias 14 (FIG. 2). As a result, the shelf 16 is an impediment to the WLI measurement of the portion of the bottom surface 18 of the via under the shelf because its view is obscured to the overlying scanning objective 20, as illustrated in FIG. 3.
The conventional approach has been to scan through the height of the via and obtain its dimensions by identifying the position of maximum fringe contrast for each pixel by some method, such as the center of gravity approach. That is, the detector pixels recording light irradiance received from the top surface of the PCB will produce a correlogram with local maximum contrast at corresponding scan positions; the pixels recording irradiance received from the bottom surface of the via visible from the top will produce another correlogram with a local maximum contrast at scan positions corresponding to the bottom; and, similarly, the surface of the fibers constituting the intermediate annular shelf produces a correlogram characterized by well-defined fringes at the scanning positions corresponding to the shelf height. Because the portion of the via's bottom surface below the fiber shelf is obscured to the interferometer's objective by the overlying shelf, no fringes have been expected to be produced by the bottom regions under the shelf. Therefore, any modulation detected by detector pixels corresponding to these bottom regions of the via has been considered noise and disregarded or treated as insignificant by the algorithms used to profile the bottom of the vias. As a result, the geometry of the bottom surface of vias has been measured based only on the information obtained from detector pixels corresponding to the visible portion of the surface (that is, the portion that is not obscured by the fiber shelf).
It is clear that the conventional approach leads to incorrect measurements because it is known that the bottom of vias is larger than the visible portion inside the inner perimeter of the fiber shelf above it. The present invention provides a simple solution to this problem.